Wireless communication systems typically include a mobile switching center (MSC) and a plurality of base stations connected to the MSC. Each of the base stations provides wireless communication services to mobile stations within geographical coverage areas referred to herein as cells, wherein a mobile station is a transceiver to which a user or subscriber of the wireless communication system is associated. Associated with each wireless communication system are frequency spectrums over which user traffic may be transmitted between the base stations and the mobile stations. The amount of user traffic transmittable over a frequency spectrum is limited, in part, by the bandwidth of the frequency spectrum.
To more effectively utilize the frequency spectrum, wireless communication systems generally have incorporated multiple access techniques, such as frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). In FDMA and TDMA based systems, the frequency spectrum is partitioned into sets of narrow frequency bands (e.g., 30 kHz). In FDMA based systems, each narrow frequency band is used to define a traffic channel over which user traffic may be transmitted between a base station and a mobile station. In TDMA based systems, a traffic channel is defined by a narrow frequency band with time slots assigned for individual call connections. Thus, in TDMA based systems (as opposed to FDMA), more than one traffic channel may be defined using the same narrow frequency band. Typically, in FDMA and TDMA based wireless communication systems, base stations use distinct and unique narrow frequency bands to reduce co-channel interference.
In CDMA based wireless communication systems, the frequency spectrum typically includes only one wide frequency band, typically having a bandwidth of 1.25 MHz or wider. A traffic channel is defined, in part, by the wide frequency band and unique codes associated with individual users or subscribers. Although traffic channels in neighboring cells may use the same wide frequency band, co-channel interference (between base stations in neighboring cells) is reduced as a result of spreading gain attributable to the unique direct sequence codes which define the traffic channels. If the frequency spectrum is partitioned into two or more wide frequency bands, co-channel interference may further be reduced by using different wide frequency bands to define traffic channels in neighboring cells.
Regardless of the multiple access technique employed by the wireless communication system, a traffic channel needs to be assigned to a mobile station before any telephone call is placed by or to the mobile station. Specifically, the traffic channel is assigned to the mobile station by the base station associated with the cell in which the mobile station is physically located. As long as the mobile station stays within the same cell, the mobile station may use the same traffic channel for the duration of the telephone call. If the mobile station moves to another cell during the telephone call, a hand-off will be performed between the base station associated with the new cell and the base station associated with the old cell. Generally, hand-offs involve the mobile station detecting a strong RF signal from a new base station, the new base station assigning a new traffic channel to the mobile station, and the mobile station switching from the old traffic channel to the new traffic channel.
Hand-offs in which a mobile station switches between traffic channels defined using the same frequency band are referred to herein as intra-frequency hand-offs. Such hand-offs are utilized in CDMA based wireless communication systems in which the traffic channels involved in the hand-off are defined using the same wide frequency band. By contrast, hand-offs in which a mobile station switches between traffic channels utilizing different frequency bands are referred to herein as inter-frequency hand-offs. Inter-frequency hand-offs are utilized in FDMA and TDMA based wireless communication systems. Inter-frequency hand-offs may also be used in CDMA based wireless communication systems in which the traffic channels are defined utilizing more than one wide frequency band.
One concern associated with inter-frequency hand-offs is a tendency for the mobile station to oscillate between two frequencies. This occurs when a mobile station first hands-off to a new frequency and then subsequently returns to the old frequency. Such a situation is referred to herein as "ping-ponging." In geographical areas where the old and new frequencies are characterized as having strong RF signals, ping-ponging is a potential problem.
Another concern associated with inter-frequency hand-offs is dropped calls. Specifically, the success rate of inter-frequency hand-offs may suffer if the mobile station is unable, for a variety of reasons, to tune to the new frequency (used in defining the new traffic channel), thereby resulting in dropped calls. To resolve this problem in CDMA based wireless communication systems, new messages have been proposed for incorporation into the well-known IS-95 standard. The proposed new messages are described in a contribution entitled "Inter-frequency Hard Hand-off Improvements (Rev. 2)," to TR45.5, TR45.5.3.1/97.03.20.02, submitted on Mar. 20, 1997, by Edward Tiedemann and Tao Chen, and a contribution entitled "Proposed IS-95B Text for Inter-Frequency Hard Hand-off Improvements," to TR45.5, TR45.5/97.03.20.03, submitted on Mar. 17-21, 1997, by Pradeep Jain, et al.
The proposed new messages include fields that instruct a mobile station to perform a periodic search in a new frequency band for a strong RF signal whenever the mobile station nears the border of two or more cells and/or sectors. Performance of such a periodic search requires the mobile station to reallocate its resources and tune between the old frequency and the new frequency. Specifically, the mobile station may dedicate a periodic block of time to tune to the new frequency and perform its search for new base stations using the new frequency. This adversely affects the mobile station in two manners. First, while the mobile station is tuned to the new frequency, the mobile station cannot search for new base stations using the old frequency. Thus, the mobile station's search for new base stations in the old frequency is hindered. Second, fewer blocks of time are available for the transmission of user traffic (over the old frequency), which may result in a degradation of perceived communicated voice quality at the mobile station.
Therefore, there exists a need for improving inter-frequency hand-offs by minimizing oscillating inter-frequency hand-offs between existing and new frequencies, and by preventing unnecessary tuning to new frequencies to search for RF signals.